Multimode optical fibers are used in many applications especially for delivering optical power. One of the most interesting applications is the so called fiber laser. In the case of a fiber laser the double-clad fiber (DCF) concept is mostly used. A DCF consists of a core to guide the laser light, surrounded by a cladding to guide the multi-mode pump light. Such a DCF, which is one of the possible optical target fibers, comprises a core, which has a relatively small diameter of for example 5 to 60 μm. It has a numerical aperture, which for example can be in the range of 0.03 to 0.2. The cladding has a thickness of some 100 μm and a numerical aperture of for example in the range of 0.3 to 0.7.
The core of such a double-clad fiber (DCF) can be doped with a rare-earth element and in this case can serve as the active gain medium of the fiber laser. In order to achieve a population inversion, which is necessary for the fiber laser to work, pump radiation has to be fed into the cladding of the DCF, the so called “target fiber”. But also in many other applications of these target fibers it is necessary to feed in electromagnetic radiation.
Electromagnetic pump radiation can be fed into the target fiber through an end surface or through the lateral cladding surface by either a free beam of pump radiation or through feeding fibers. From U.S. Pat. No. 5,864,644 it is known to feed in the pump radiation by feeding fibers which are usually bundled together, drawn into a taper and fusion spliced to an end surface of the target fiber. For fiber laser applications in the middle of the fiber bundle a signal fiber has to be placed to feed in or out the laser light. This technique is known as Tapered Fused Fiber Bundle (TFB) technique. This technique has two major disadvantages. The first disadvantage is the splice between the tapered fibers and the target fiber, where the fibers to be connected have to be aligned very exactly. For example, a small misalignment can lead to a strong degradation of the beam quality of the laser. The second disadvantage is the tapering process of the feeding fibers and the signal fiber to match the optical requirements of the core and the cladding of the target fiber. For example, a small taper mismatch can lead to a significant thermal load of the fiber coupler and also to a strong degradation of the beam quality of the laser. Another disadvantage is that due to existence of only two target fiber end surfaces the number of pump feeding fibers is limited. Thus, the laser power can not be simply scaled up by using more pump diodes.
These disadvantages can be overcome by feeding in the pump radiation through the cladding surface. The cladding surface is the lateral surface of the target fiber extending in longitudinal direction of the target fiber. The feeding of pump radiation through the cladding surface is called non-axial transfer. In order to achieve this, usually at least one feeding fiber is fused to the target fiber such that the cladding surfaces of both the feeding fiber and the target fiber are in optical contact with each other. To achieve an even better coupling of the pump radiation to the target fiber it is known to use feeding fibers having a converging taper portion. This means the diameter and thus the size of the cross-section of the respective fiber is progressively reduced. Using a converging taper portion of the feeding fiber is known from U.S. Pat. No. 5,999,673 and U.S. Pat. No. 7,933,479 B2.
Due to losses and inefficiencies not all of the pump radiation can be transferred from the feeding fibers into the target fiber. This lost power results in a heating of the coupling arrangement and thus limits the maximum amount of radiation power that can be transferred through a coupling arrangement. It is therefore advantageous to reduce the amount of radiation power that is not transferred into the target fiber and to ensure that this portion of the radiation power does not result in a strong heating of the coupling arrangement.
In order to achieve an optimal coupling result the numerical aperture of the feeding fiber should be smaller than or equal to the numerical aperture of the target fiber. For both the feeding fiber and the target fiber, the relevant numerical aperture is the one of the outmost layer, guiding the multi-mode pump light, of the respective fiber. For the feeding fiber this usually is the core of the feeding fiber. Hence, for better coupling results it is advantageous to remove the cladding of the feeding fiber. This unfortunately in practice is rather difficult, since in many cases both the core and the cladding consist of almost the same material differing only by some doping elements. The cladding material is often glass in order to be able to transport high radiation power through the feeding fiber and the fiber can easier withstand the deleterious effects of high temperature and humidity encountered during operation. Hence, it is difficult to remove the cladding for example using an etching technique, and stop the etching when the cladding is fully removed and the core has not been damaged. Removing the coating of the target fiber and the feeding fiber is in many cases much easier, since the coating often consists of a polymer, which can easily be removed.
In order to get good coupling results and a large radiation transfer from the feeding fiber into the target fiber even if the feeding fiber still comprises a cladding the taper-ratio has to be increased. The taper-ratio is the ratio of the diameter of the feeding fiber at the beginning of the converging taper portion and the diameter of the feeding fiber at the end of the converging taper portion. Hence, a taper-ratio of 4 means, the diameter of the feeding fiber is reduced in the converging taper portion to one forth of the original value. Increasing the taper-ratio leads to a better transfer of electromagnetic radiation from the feeding fiber into the target fiber but reduced the mechanical stability of the coupling arrangement.
A coupling arrangement as described above is usually manufactured by mounting the involved fibers in a mechanical mount and twisting the at least one feeding fiber around the target fiber. By applying a force in longitudinal direction the converging taper portion and the diverging taper portion are generated by heating of the twisted fibers. Since the fibers touch each other during the twisting, the feeding fibers get fused to the target fiber. Unfortunately the target fiber also shows a converging taper portion and a diverging taper portion after an arbitrary longitudinal irreversible extension, leading to changed optical characteristics of the fiber. In order to reduce the tapering of the target fiber as much as possible it is known from U.S. Pat. No. 7,933,479 B2 to perform a pre-tapering of the feeding fibers. This means the feeding fibers are heated and subjected to a force applied in longitudinal direction before they are mounted in the mechanical mount and twisted around the target fiber. The feeding fibers then comprise a converging taper portion and a diverging taper portion when they are mounted and thus, the extension in longitudinal direction of the twisted fibers can be reduced leading to a reduced tapering of the target fiber. The disadvantage is that the pre-tapered feeding fiber becomes very damageable due to the reduced diameter.
There is thus a need to provide coupling arrangements having a better coupling efficiency, providing better mechanical stability and being capable of transferring higher radiation power. There is further need of a method to manufacture such a coupling arrangement in an easy, cheap and reproducible way.